1. Field of the Invention
The present invention relates to a process for the treatment of pyrophoric elemental phosphorus (e.g., P.sub.4 or P.sub.2)-containing material.
2. Description of the Prior Art
The treatment and disposal of pyrophoric elemental phosphorus-containing material is of serious concern to persons of ordinary skill in the art. Since such material is highly pyrophoric, due to the pyrophoric nature of the elemental phosphorus contained therein, stringent precautions need to be taken to protect the material from contact with oxygen in the atmosphere (e.g., by blanketing the material with a protective layer of water). Examples of such pyrophoric elemental phosphorus-containing materials are described hereinafter. Contact of such pyrophoric materials with the atmosphere will cause spontaneous fire which is extremely difficult to suppress due to the very high temperatures produced. Hence, in accordance with the present invention, the necessity of a protective blanket (e.g., of inert gas or, preferably, water) over the material, even though the present invention involves contacting or sparging of the material, while so protected, with an oxygen-containing gas.
For example, of concern to persons of ordinary skill in the art is the problem of safely disposing of the extremely pyrophoric, substantially solid P.sub.4 -containing waste which is usually found in appropriate storage tanks under an aqueous protective layer. This solid material often forms from the more flowable P.sub.4 -sludge to be described hereinafter if it is allowed to stand for a duration of time. Such substantially solid waste material generally has a very high P.sub.4 content (e.g., greater than about 90%). Conventional means for removal of such a waste product has involved the laborious fracturing (e.g., by use of a jackhammer) of the solid formation, while under a protective water cover, and its subsequent removal in small containers, also while being shielded from contact with the air.
Also, the production of elemental phosphorus, for example, by the smelting of phosphate rock, often results in the production of a certain quantity of a more fluid sludge which comprises elemental phosphorus droplets or globules, solid impurities and water. The amount of phosphorus sludge (e.g., P.sub.4 -sludge) that is produced will vary depending upon numerous factors such as the initial composition of the rock charge, and the operating conditions and the design of the furnace. Phosphorus sludge can contain anywhere from about 5%-90% or more by weight of elemental phosphorus. The phosphorus sludge that is produced can vary from about 10 to about 60 weight percent or more of the furnace output of elemental phosphorus.
As is known in the art, the sludge that is produced is an undesirable by-product of the process. Since the sludge contains elemental phosphorus values, it is desirable that it be treated to recover the phosphorus values in some usable form. Phosphorus is pyrophoric. This quality, as mentioned before, also makes the sludge by-product quite dangerous since exposure to air can result in sudden pyrophoric incidents and lead to fire and possible damage to property and/or injury to persons. Therefore, a need has existed in the art for ways in which this type of sludge can be treated to either render it less pyrophoric and/or to recover phosphorus values therefrom.
Various methods have been proposed to treat phosphorus sludge in order to either convert the phosphorus therein to a more desirable form and/or to recover phosphorus values therefrom. Among these are such physical methods as filtration, distillation, stirring and settling, vibration, centrifuging, briquetting, and the like. There have also been various chemical innovations for recovering or treatment of phosphorus in sludge in the literature.
For example, U.S. Pat. No. 3,084,029 disclosed that when small amounts of dispersing agents are introduced into the sludge and the pH of the sludge is adjusted to about 6, the viscosity and stickiness of the sludge was reduced.
U.S. Pat. No. 3,104,952 proposed mixing the sludge with phosphoric acid and steam distilling the mixture in a non-oxidizing atmosphere.
U.S. Pat. No. 3,442,621 proposed treatment of sludge with chromic acid and the coalescing of the phosphorus in the sludge in the presence of an acidulating agent in order to decrease the phosphorus content of the sludge.
U.S. Pat. No. 3,615,218 taught recovery of phosphorus values from sludge by admixing the sludge with carbon disulfide.
The procedures shown in each of the previously described U.S. Patents are distinctly different from the approach utilized in connection with the present invention.
An oxidizing agent was proposed for use in U.S. Pat. No. 3,436,184 to decrease the phosphorus content of the sludge. Although this patent taught that any oxidizing agent could be used, it named only oxidizing agents which were either chemical compounds or, in the case of the halogens, elements. The disclosure of this patent contained no clear teaching or suggestion of using the particular type of treatment agent (i.e., an oxygen-containing gas) that is the subject of the present invention.
More recently, U.S.S.R. Pat. No. 697,401 teaches the treatment of phosphorus-containing sludges in a fluidized bed at high temperatures. This patent generally teaches heating the "P-containing sludge" at temperatures of from 44.degree.-90.degree. C. while being thoroughly mixed by bubbling an oxygen-containing gas through it for 30-60 minutes. The resulting composition is then treated in a fluidized bed at 120.degree.-180.degree. C. for 1-2.5 hours with a fluidizing agent velocity of 3-5 meters per second. This reference is utterly silent regarding blanketing its sludge layer from contact with oxygen in the atmosphere. Example 2, which relates more specifically to treatment with an oxygen-containing gas, first "neutralizes" its waste water product with milk of lime to yield a lime sludge which is then heated and treated with the oxygen-containing gas. The present invention appears to differ from this disclosure in that the present process is practiced while the elemental phosphorus-containing material (which is not first neutralized) is otherwise protected from contact with oxygen in the atmosphere by being blanketed with a non-flammable fluid (e.g., by being protected with a blanket layer of water).